DNA ploidy and cell proliferation were studied by means of flow cytometry in 98 patients with primary colorectal adenocarcinoma. Multiple samples of tumour burden were pooled and freshly dissociated immediately after surgery for FACS analysis. The relationships between ploidy, proliferative activity, evaluated in terms of S-phase percentages (%S), and some clinico-pathological variables were analyzed. 87 of the 98 tumors yielded evaluable DNA histograms: 32 were diploid (37%) and 55 were aneuploid (63%; median DNA index = 1.6). Multiple aneuploid cell populations were found in 15 tumors (17%). The % S was estimated by means of a mathematical model. Aneuploid tumors showed % S values significantly higher than diploid ones (p < 0.0001). Differences in the distribution of DNA aneuploidy were observed in relation to Dukes' stage and tumor site, left colon, rectum and stage D tumors being more frequently aneuploid. No significant differences in proliferative activity were observed in relation to most of the clinical variables, except for higher % S values observed in tumors of right colon compared to those of left colon and rectum.